In order both to speed up the growth process of plants and to provide a year round supply for the consumer, many vegetables are grown either completely or partially isolated from the outside environment, for example in greenhouses. The conditions under which the plants are grown can thereby be closely regulated and parameters such as temperature, water, nutrient make-up, light etc. carefully controlled. The water and nutrient level can especially be controlled where a substrate other than soil is used for the plants to grow in, for example rockwool, perlite, silica gel, and plastic foams. These do not actually supply the plant with minerals but serve simply as a substrate for the root system. In such so-called hydroponic systems, the nutrients such as potassium, sodium, molybdenum, phosphate, nitrate, etc. are usually applied dissolved in the water. Although this can be carefully controlled, some excess water supply will inevitably occur. The excess can then either be discharged to the environment or recycled, possibly with further treatment before re-use. The former option is now less favoured for the following reasons. First, plant nutrients dissolved in the water can cause eutrophication. Secondly, the water may in addition contain pesticides which could also have a negative impact on the environment if discharged in large quantities. Thirdly, it is becoming increasingly undesirable to discharge water to drain, not just on cost grounds but also to conserve a restricted supply. Re-use of the excess water does however bring with it its own problems, especially where the water is utilised for the same plants. The chief reason for this is that plant pathogens can be carried through in the water and the irrigation system then often provides suitable breeding conditions for the plant pathogen, which makes re-use of this water highly unsuitable. It is often desirable therefore for a suitable disinfectant to be added at some point in the water recycle to remove or reduce the pathogen population. Well known disinfectants in this area include hypochlorite, ozone and UV irradiation as well as biological control mechanisms. A further group of compounds suitable for disinfecting the substrate and the water supplied to the plant is disclosed in British Patent Application 2,224,441, where organic percarboxylic acids, particularly peracetic acid are used.
Simply killing any plant pathogens in the water via disinfection does however have an unwanted side-effect due to the biological equilibrium of the water being thus disturbed. According to the Derwent.RTM. abstract of Danish Patent Application DK 9300538 the population of one particular group of fungal species Trichoderma spp. is normally kept down due to competition from other micro-organisms present in the water. When these competitors are removed however, for example by disinfection, then the population of the Trichoderma can increase. It can eventually form a fibrous mat which can block up pipework and nozzles used to irrigate the plants. Removal of this material is a time consuming process, involving temporary dismantling of the pipework of the irrigation system, which would, and especially in warmer weather, need to be carried out frequently.
In the above-mentioned disinfectants which can, potentially, be used, the following disadvantages may be mentioned. Hypochlorite suffers from two main problems. First, the solutions containing the hypochlorite will also comprise a large quantity of sodium chloride which may not be suitable for the particular plant grown. Secondly, hypochlorite is increasingly coming under pressure from environmental considerations due to their capacity to form unwanted chlorinated organic compounds via side reactions. For ozone and UV treatment, expenditure and equipment can be expensive.
Biological control is also difficult to achieve as care must be taken, first to select a control which can adequately maintain the targeted micro-organism(s) at an acceptable level, and secondly to select a control which will not cause problems itself--for example by reaching an unacceptable population size.
The above problem with Trichoderma can be exacerbated when percarboxylic acids are used. Percarboxylic acids are usually supplied as an equilibrium solution containing the parent carboxylic acid, and furthermore once the percarboxylic acid has carried out its disinfection then the carboxylic acid remains as a by-product. As some carboxylic acids can be readily used as a food source by Trichoderma the result is that although its population may initially drop, the end result of application of the percarboxylic acid can be an increase in Trichoderma's population.
Potential problems with percarboxylic acids have been further exemplified in the prior art when they have been applied to such recycle systems, for example in the Derwent.RTM. abstract of the Dutch Patent Application 9201631. In this patent percarboxylic acids were used to control bacteria and other micro-organisms. A further treatment was however also required using ultra-violet radiation to kill off viruses and moulds, along with occasional treatment of the water with acidified percarboxylic acids to remove various deposits which built up during prolonged use of the irrigation system.
Conventional recycle systems are particularly prone to fouling by unwanted microbial species. Especially during periods of low water uptake by the plants, large volumes of water remain stagnant, and hence provide an excellent breeding ground for species such as Trichoderma. Although much research, as outlined above) has been carried out into methods of disinfection of the irrigation water, the aforementioned problems have not been solved.
It is therefore an object of the present invention to solve these problems by providing a process for hydroponic cultivation of plants which makes it possible to control plant pathogens whilst at the same time inhibit the selective growth of other non pathogenic microbial species such as Trichoderma spp.